Class 3 semaphores (Sema3) are repulsive cues that are involved axon guidance during development of the nervous system. Their continued expression in the adult central nervous system (CMS) as well as their upregulation at the lesion site after spinal cord injury (SCI) raises the possibility that they may contribute to the failure of axonal regeneration after SCI. We will investigate this possibility by using mutant mice that lack the signal transducing moiety of SemaS receptor complex, namely the Plexin A3 and A4 subtypes. The specific aims of this proposal are as follows: 1) Investigate the effect of disrupting class 3 semaphorin signaling on regeneration of the corticospinal, serotonergic, and ascending sensory axons after a complete mid-thoracic transection. 2) Investigate the effect of disrupting class 3 semaphorin signaling on the formation the astrocyte-meningeal fibroblast border. 3) Generate a conditional Plexin A4 knockout mouse and analyze the regenerative phenotype of its 5-HT axons after acute deletion of Plexin A4 in astrocytes. This research will aid in better understanding how chemorepellents can contribute to axon regeneration failure, and how overcoming this inhibitory barrier may be used to treat spinal cord injured patients. The results of this research will increase our understanding of how molecules that are important for repelling axons during development can negatively affect axonal regeneration in the adult mammalian central nervous system. Overcoming this inhibitory barrier to enhance axonal regeneration can have significant therapeutic implications for patients who suffer from spinal cord injury.